(a) Field of the Invention
This invention relates to an electrophotographic toner, specifically, to a electrophotographic toner which exhibits good fixation in high-speed copying by the heat fixing method and moreover is not affected by an environment such as high humidity and hence has high durability.
(b) Description of the Prior Art
Plain paper copying machines making use of electrophotography the like and high-speed printers for computer terminals, which rely upon the same principle, have found wide-spread commercial utility in recent years because their ease in operation and their ability in providing copies of excellent quality have been rated high. In the meantime, consistent efforts have been made with a view toward making the copying speed still higher and reducing the machine maintenance work as much as possible. Such improvements have been made not only in copying machines but also toners. Reflecting the improved copying speed, a toner is required to have such properties that toner images can be fixed at a temperature as low as possible. The toner is also required to exhibit anti-offset properties over a temperature range as wide as possible. With such required properties in view, properties of a resin contained in a toner are considered as the most important parameters for obtaining marks of excellent quality consistently during a continuous copying operation without being affected by various environmental conditions.
With a target on such improvement of a resin, it has been known from U.S. Pat. No. 3,853,778 to use a crystalline polymer having an amorphous backbone and side-chain crystallinity derived by the polymerization of a monomer having a crystalline alkyl group of at least about 14 carbon atoms. According to the above patent specification, it is indicated that a resin composition obtained by copolymerizing about 70 mole % of docosyl acrylate, which contains a long-chain alkyl group of 22 carbon atoms, or di-n-docosyl fumarate or the like with about 30 mole % of styrene or the like is permanently fixed by its heat fixing at 65.degree. C. for 10 seconds. However, use of such monomers leads to an unavoidable increase in the fixing temperature, thereby failing to achieve good continuous copying performance and at the same time, reduces the blocking resistance significantly. Even when fumaric acid is used instead of di-n-docosyl fumarate, a similar tendency is exhibited in connection with fixation and the triboelectricity is increased. These polymers have thus been found unsuitable for high-speed copying.